Conventionally, in regard to control of an internal combustion engine for an automobile, torque demand control has been known, which controls operations of actuators such as a throttle and an ignition device to realize a required torque. For example, Japanese Patent Laid-Open No. 2006-200466 and Japanese Patent Laid-Open No. 2009-047101 describe the inventions relating to such torque demand control.
In the torque demand control of an internal combustion engine, integrated control of each of the actuators relating to the operation of the internal combustion engine is performed. In the case of a spark ignition type internal combustion engine having a throttle, torque can be controlled by integrated control of the throttle and an ignition device. However, in the process of the integrated control, the operation states of the other actuators need to be taken into consideration. More specifically, in the case of an internal combustion engine having a variable valve timing mechanism (hereinafter, described as IN-VVT) which changes the valve timing of an intake valve, the respective operation amounts of the throttle and the ignition device need to be determined with the operation state of the IN-VVT taken into consideration. This is because the valve timing of the intake valve affects an internal EGR, and the internal EGR has an influence on the torque of the internal combustion engine.
The influence which the operation state of an IN-VVT has on torque will be described more specifically. When the IN-VVT is at the maximum retardation position, the valve overlap of the intake valve and the exhaust valve does not exist, and the internal EGR becomes the minimum. Therefore, if the ignition timing is constant, the torque becomes the maximum when the IN-VVT is located at the maximum retardation position. As the valve timing of the intake valve advances more, the influence of the internal EGR becomes larger, and the torque becomes smaller than the maximum torque. FIG. 6 shows the state by a pressure-crank angle diagram. In FIG. 6, changes of the cylinder pressure after the ignition timing shown by a star mark are shown by being compared between the case in which the valve timing of the intake valve is the maximum retardation, and the case in which the valve timing is advanced by 20 degrees in the crank angle from the maximum retardation position. Here, the time when the IN-VVT is at the maximum retardation position is set as a reference, and the valve timing (INVT) of the intake valve at this time is set as 0 degrees. As shown in FIG. 6, by advance of the valve timing of the intake valve, a delay occurs in the combustion center, and as a result, the maximum value of the cylinder pressure is reduced. The delay of the combustion center is due to reduction of the combustion speed by increase in the EGR rate. Torque is correlated with the maximum value of the cylinder pressure, and therefore, reduction in torque occurs by the combustion center being delayed by advance of the valve timing of the intake valve.
However, in the ranges of the literatures which are known to the public at the present point of time, the description relating to the integrated control which takes the operation state of an IN-VVT into consideration is not found. Thus, one method of the integrated control which was studied in the inventing process of the present invention will be introduced.
The studied method is the method which reflects the operation state of an IN-VVT, that is, the valve timing of the intake valve in the ignition timing control. In this method, the valve timing of the intake valve is not taken into consideration in the process of calculating a target air quantity from a required torque. The air quantity required for realization of the required torque is calculated on the precondition that the valve timing of the intake valve is at the maximum retardation, irrespective of the actual valve timing of the intake valve, and the throttle is controlled with the calculated air quantity as a target air quantity. Subsequently, the ignition timing is corrected by being advanced in accordance with the actual valve timing of the intake valve to compensate reduction of the torque due to advance of the valve timing of the intake valve. The required torque has been expected to be realized without being influenced by the operation state of the IN-VVT by integrally controlling the throttle and the ignition device by the method like this.
However, it is actually difficult to realize the torque as required with only correction of the ignition timing. FIG. 7 shows the correction result of the ignition timing according to the aforesaid method by a pressure-crank angle diagram. As shown in FIG. 7, when the ignition timing is corrected by being advanced in the state in which the valve timing of the intake valve is advanced (VT=20 in FIG. 7), the maximum value of the cylinder pressure exceeds the cylinder pressure which is sufficient for realization of the required torque. More specifically, torque is excessively outputted to exceed the require torque. This is considered to be because increase in the internal EGR due to advance of the valve timing of the intake valve leads to various factors to increase the torque, such as reduction in pumping loss, increase in the compression end pressure, or reduction in cooling loss.